In most bicycles, the front wheel is mounted on an axle that extends transversely between the distal ends of a pair of fork legs. The fork legs are joined at their upper ends by a crown, and a steering tube extending up from the crown is received in a headset, by which the fork is joined to the frame for steering rotation. The headset includes upper and lower ball bearings supported within in a head tube of the frame and supporting the steering tube within them. The handlebar post is received within the steering tube and is affixed to it, usually by a cam type locking mechanism.
Ordinary ball bearings carry loads that act perpendicularly to the axis of rotation of the shaft. Accordingly, torques applied to a shaft mounted in them in a directions tending to skew the shaft resolve as forces on the bearings that are a function of the distance between the bearings. The diameter of the bearings is of little consequence to the bearing loads due to torques on the shaft tending to skew it. In conventional headset designs, therefore, it is suitable to have a head tube of a relatively small diameter. On the other hand, the axial length of the head tube and the axial spacing of the headset bearings should be as large as possible to carry lateral torque loads on the steering tube but are in practice limited by the bicycle size and geometry and are generally small. Accordingly, bearing loads due to cocking of the steering tube within the headset bearings under steering loads, braking loads, and bumps to the front wheel are high.